There is a buy/sell/trade forum that all sales on this forum must go through. You need 100 posts to view and post in it. Trying to sell outside of that forum will result in your thread being locked, as has happened before.
thats nice, cause i also wanted to sell something, well one of my n3ds since i recently caved and bought a us n3ds. so trying to sell my jp n3ds @ 9.2. cant wait before i have access to that board. so i can post, man i splurged, 4 3ds, 3ds, 3dsXL, N3DSXL JP, and N3DSXL US. LOL! waste of monehhhhhhhhhhhh
Since no one actually addressed your question, assuming you mean your flashcard, yes it is. Just don't put games on it (if it has built in memory and/or an included microSD card). Then you'd be selling pirated copies of games, and that's a big no-no.
Then yes, I don't see why you're not allowed to sell here, though there is a possibility of the thread getting locked as others have stated.
If you don't have any luck here then maybe the official gw forum will help, theres a buy/sell section there where I bought my housemate his gw for a reduced price.